A whole onion stuffed with mashed potatoes would be a striking all-in-one side dish for steak, I decided. So that was on the menu, only it turned out that most of my bag of onions (another recent Costco purchase and probable victim of freezing) were mostly bad. The only one that wasn't was HUGE. So in place of individual stuffed onions, I halved the one, removed all but the outer five layers (that left a 1" shell), and roasted them in butter cut side down (about 20 minutes at 400F gave them nice color) before filling them with a mixture of potatoes, butter, milk, chives, cumin and smoked spanish paprika.
Kind of like a stuffed baked potato, only now the shell was an onion.
I made these earlier in the afternoon with a plan to microwave them to a finish at dinner time. The flavoring was part of a hankering that developed over the day for a suite of Southwest flavors that was going to include grilled green onions, an avocado compound butter and squeeze of fresh lime on the steak. But by dinner time my emphasis had shifted from food to wine, and I decided the green stuff was all wrong for the probably-delicate 79 Bordeaux we selected for dinner so I reverted to my original plan for the steak which was to coat it with olive oil and smashed garlic, salt and pepper.
The result was good but not great, attractive but not stunning. And flavorwise, there was no synergy. The two didn't meld or compliment one another or, in combination, the steak in a way that made the combination as compelling as either only onion or only potato would have been. The result, therefore, merely seemed affected.
Worth exploring for some other meat or preparing on it's own as a vegetarian main course? No, unfortunately, I really don't think so.